A tree may acquire much more than wood during its long life.
Many a chain saw has been ruined, and sometimes people are hurt, when pruning or removal tools come up against hidden hardware from long forgotten repairs to a special tree or concrete filling dating ‘way back to the unenlightened times. Nails may be embedded, last vestiges of a sign that once proclaimed the area to be the "Boys Club". A wire fence that once ran behind the trunk may be gone without visible trace yet the tree hides several squares -- a bit cut out by the demolition team because it had been grown over by the tree and is now totally engulfed. Fast growing “weed” trees often hide such hazards -- sections of utility cable, hammock hooks, entire metal signs and even fenceposts can end up within the wood.
Marbles, polished stones and toys left in crotches can be completely grown over and although some might discover them and say, "Aww, remember when…", others say "$@#&*" as they cause a power saw to buck or the teeth of a fine hand saw to twist out of line.
If you know the site history, warn the arborist about old attachments to trees. Or, when metal first screams on metal to remind you, speak up! Say, “Oh, right! There were four of those hooks in that tree, probably about here, here and here.”